It is common knowledge that in a conventional television broadcast system color control is a function of the phase and amplitude of the chrominance signal. More specifically, R-Y and B-Y signals generated from a camera matrix modulate respective 3.58 MHz sub-carriers which are phase shifted from each other by ninety degrees. These modulated signals are added vectorally to generate a chrominance signal having a phase angle which represents the color or hue information of the image being transmitted, and having an amplitude which represents the color saturation information.
Since color is represented by the phase angle of the chrominance signal, hue adjustment is often accomplished in a conventional television receiver by changing the phase relationship between the chrominance signal and the local 3.58 MHz color sub-carrier. This phase angle change generates changes in the amplitudes of the demodulated R-Y and B-Y components of the chrominance signal.
In a non-broadcasting video system, such as in a closed-circuit video where the signal is fed to remote units by wire, there is no need to generate a chrominance signal. Rather, the color signals out of the camera may be fed directly to an image display terminal. However, with the absence of phase angle information which is present in the chrominance signal, means other than phase angle adjustment are necessary in the closed circuit in order to control the color hue.
A number of conventional color adjusting devices have been disclosed. For example, a color correction circuit is set forth in U.S. Pat. No. 3,729,578 by Slusarski wherein R-Y and B-Y signals are clipped, and the clipped portions are then added to or subtracted from the R-Y, B-Y and G-Y signals to produce the desired color changes.
Weitzsch, in U.S. Pat. No. 3,689,689 discloses a color correction circuit whereby skin color and green intensity are adjusted by changing the color signal voltages.
Color correction is also disclosed in U.S. Pat. No. 4,219,840 by Srivastava which utilizes a color modifier which reduces the gain of the B-Y signal and adds a -(B-Y) component to the B-Y demodulator output.
Color correction is further disclosed by Moore in U.S. Pat. No. 3,749,825 whereby portions of the R-Y and G-Y signals are summed with the B-Y signal to achieve the desired color change.
In U.S. Pat. No. 4,633,299 by Tanaka, color correction is accomplished by detecting the color saturation level of a color signal and increasing a color component on the screen in response to the detected saturation increase.
A circuit for adjusting color hue by rotating the vector sum of the color mixture signals is disclosed in U.S. Pat. No. 4,562,460 by Harwood.
Freyberger et al., in U.S. Pat. No. 4,568,967, discloses a digital color signal processor which multiplies two demodulated digital color difference signals by a previously multiplied digital color saturation signal, to generate three time division-multiplexed signals which are added to the luminance signal and then processed to provide the analog color signals.
A digital color hue adjustment circuit disclosed in U.S. Pat. No. 4,542,402 by Ader, includes stored coefficients which are multiplied by the color difference signals; the stored coefficients being modified by a microprocessor in response to adjustment of contrast and tint controls.
Hue correction for a digital television is also disclosed in U.S. Pat. No. 4,558,351 by Fling et al., whereby signals derived from I and Q inputs are multiplied by stored hue correction coefficients to produce corrected I and Q signals.
Other digital color circuits have been disclosed in U.S. Pat. No. 4,568,968 by Ullrich (color television matrix circuit); U.S. Pat. No. 4,644,389 by Nakagawa et al. (digital hue correction); and U.S. Pat. No. 4,550,339 by Fling (digital hue correction by changing chrominance vector angles).